DRAINAGE AND VENTILATION 53 



constant interchange of gases in the soil and the air above by the 

 principle known as diffusion. The difference in temperature de- 

 veloped between the top soil and subsoils also assists in this process. 

 Every change which takes place in the atmospheric pressure above 

 a field has a tendency to cause some air to pass either into or out 

 from the soil. 



Under the influence of a heavy wind which comes in puffs, 

 there is a tendency to alternately force such air out from the soil 

 pores and again to allow it to return, thus producing an irregular 

 but strong soil respiration. This is especially noticeable in Okla- 

 homa, Texas, and some of the western States where there are high 

 winds. As proof of this movement of the air we might call atten- 

 tion to the fact that in certain sections of the country which are 

 underlaid by extensive beds of coarse gravel the wells sunk into 

 these beds are often subject to strong draughts which alternately 

 pass into and out of them. Prairies that are underlaid with layers 

 of coarse gravel are usually very productive on account of the strong 

 soil ventilation. It is claimed that in underdrained fields where 

 lines of tile are placed three to four feet deep and from fifty to 

 one hundred feet apart, there is provided a ventilation system 

 which greatly facilitates the exchange of soil air. 



Overventilation. While some soils are underventilated, it fre- 

 quently happens that other soils are so open that they are over- 

 ventilated and the soil moisture is rapidly exhausted. Air and 

 water both pass freely through this kind of soil and bear away nearly 

 all the plant food. Such soils should receive shallow tillage and 

 the undersoil should be kept firm. Applications of farmyard 

 manure which tends to clog the pores of the soil will be found 

 beneficial. Again, when the winds are unusually high, there is a 

 heavy loss of the fine dust particles from light soils. This waste 

 accumulates from day to day and soon impoverishes the soil, be- 

 sides decreasing its water-holding power. 



This destructive action of the wind is especially noticeable in 

 arid and semiarid regions. In Wisconsin there are extensive areas 

 of light lands where west or northwest winds after storms often 

 sweep entirely away crops of grain, even after they are four or 

 five inches high. In western Oklahoma, Texas, and Kansas simi- 

 lar conditions often exist. 



